1. Field of the Invention
The present invention relates to an apparatus for detecting ferromagnetic materials, and particularly to an apparatus for sensing the motion of ferromagnetic bodies for the purpose of counting them.
2. Description of the Prior Art
U.S. Pat. No. 3,986,430, issued Oct. 19, 1976, describes a method of loading explosive cartridges downwardly into a borehole wherein the descent of the cartridges is guided through a retractable flexible tubing assembly through which a stream of water flows. The cartridges preferably are cut open just before their discharge from the assembly, and their passage through, or discharge from, the assembly is indicated outside the borehole. The assembly is retracted incrementally until completion of the loading.
The feature of sensing and indicating the passage of the cartridges through, or their discharge from, the cartridge-guiding assembly is important inasmuch as it provides information on whether the cartridges fed into the tubing assembly are being discharged or whether the flow has ceased due to blockage, e.g., by the buildup of cartridges in the assembly. The loading method requires a device which is (a) capable of sensing the motion of the cartridges in the environment of the borehole, e.g., in water possibly contaminated with materials such as salts; (b) compact and sufficiently simple in design to allow it to be used in conjunction with the cartridge-guiding tubing without interfering with the motion of the cartridges therethrough, and with a minimum number of connections to the surface; and (c) cheap enough to be expended when the cartridges are subsequently detonated. In addition, it is desirable that the device be functional at great depths, e.g., at 1000 feet or deeper, and be able to detect small objects moving at high speed.
The requirements of the described cartridge loading method preclude the use of various of the counting techniques known in the art. For example, permanent magnetization of a metal clip which closes the end of a chub cartridge and detection of the presence of the clip by a Hall effect device are unsuitable because the device is too expensive to be expendable, and four lead wires are required between the sensor located in the loading assembly and the electronics box on the surface. The use of an inductance coil in the assembly to sense the presence of a steel cartridge clip is more suitable with respect to expendability, but presents certain other problems associated with the detection circuit. If a conventional alternating current bridge with resistances, a use of which is described in U.S. Pat. No. 1,708,524, is used, accurate counting is hampered by the fact that no true null occurs due to the large out-of-phase signal caused by a phase shift in the inductance and stray capacitances. An impedance bridge is costly as well as troublesome in that the balancing of variable lead wire impedances requires interactive resistance and reactance adjustments.